The function of numerous proteins requires the modification of multiple glutamic acid residues to γ-carboxyglutamate. Among these vitamin K-dependent (VKD) coagulation proteins, FIX (Christmas factor), FVII, and prothrombin are the best known. The observation that a knock-out of the gene for matrix Gla protein results in calcification of the mouse's arteries (Luo et al. (1997) “Spontaneous calcification of arteries and cartilage in mice lacking matrix GLA protein” Nature 386:78-81) emphasizes the importance of the vitamin K cycle for proteins with functions other than coagulation. Moreover, Gas6 and other Gla proteins of unknown function are expressed in neural tissue and warfarin exposure in eutero results in mental retardation and facial abnormalities. This is consistent with the observation that the expression of VKD carboxylase, the enzyme that accomplishes the Gla modification, is temporally regulated in a tissue-specific manner with high expression in the nervous system during early embryonic stages. Concomitant with carboxylation, reduced vitamin K, a co-substrate of the reaction, is converted to vitamin K epoxide. Because the amount of vitamin K in the human diet is limited, vitamin K epoxide must be converted back to vitamin K by vitamin K epoxide reductase (VKOR) to prevent its depletion. Warfarin, the most widely used anticoagulation drug, targets VKOR and prevents the regeneration of vitamin K. The consequence is a decrease in the concentration of reduced vitamin K, which results in a reduced rate of carboxylation by the γ-glutamyl carboxylase and in the production of undercarboxylated vitamin K-dependent proteins.